U.S. patent number 3,569,873 [Application Number 04/776,497] was granted by the patent office on 1971-03-09 for insertion loss equalization device.
This patent grant is currently assigned to Collins Radio Company. Invention is credited to William D. Beaver.
United States Patent |
3,569,873 |
Beaver |
March 9, 1971 |
INSERTION LOSS EQUALIZATION DEVICE
Abstract
A loss equalization device comprising a piezoelectric quartz
crystal plate with two pairs of metallic electrodes deposited
thereon to form two coupled resonators. The structure is terminated
in a resistance different from the midband impedance of the
structure's passband so that there is more attenuation in the
center of the passband than in the areas off the center. By
cascading this device with a lossy, properly terminated filter,
there is obtained a substantially flat frequency response
curve.
Inventors: |
Beaver; William D. (Costa Mesa,
CA) |
Assignee: |
Collins Radio Company (Dallas,
TX)
|
Family
ID: |
25107533 |
Appl.
No.: |
04/776,497 |
Filed: |
November 18, 1968 |
Current U.S.
Class: |
333/192; 333/28R;
333/18 |
Current CPC
Class: |
H03H
9/542 (20130101) |
Current International
Class: |
H03H
9/00 (20060101); H03H 9/60 (20060101); H03H
009/00 (); H03h 007/08 () |
Field of
Search: |
;333/22,81,72,76,18,28,28 (A)/ ;333/32 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Saalbach; Herman Karl
Assistant Examiner: Chatmon, Jr.; Saxfield
Claims
I claim:
1. A loss equalizer device for equalizing the loss of a
conventional filter over a given passband and comprising:
crystal plate means;
first and second pairs of electrodes coated on said crystal plate
with each pair of electrodes comprising a coated area on
substantially coinciding opposite sides of said crystal plate;
a pair of input terminals connected to said first pair of
electrodes;
a misterminated impedance means having an impedance different from
the midband impedance of said device connected to said second pair
of electrodes to provide greater loss at the center of said
passband than at points within said passband and near the shoulders
thereof;
and means for cascading said loss equalizer with said conventional
filter.
2. A loss equalizer device in accordance with claim 1 in which said
means for cascading said loss equalizer with said conventional
filter comprises:
T network means comprising first, second, and third resistive means
forming the left half of the crossbar of the T network, the right
half of the crossbar of the T network, and the leg of the T
network, respectively;
said first and third resistive means comprising said misterminated
impedance means and being connected across said second pair of
electrodes; and
said second and third resistive means comprising the coupling
impedance to said conventional filter.
3. A loss equalizer device in accordance with claim 1 in which:
said pairs of electrodes are positioned a given distance apart and
form a pair of resonators coupled together through that portion of
the crystal plate lying therebetween;
said given distance being sufficient to substantially isolate each
resonator one from the other with respect to electrical coupling
but sufficiently close to permit mechanical coupling between said
resonators.
4. A loss equalizer device in accordance with claim 3 in which said
means for cascading said loss equalizer with said conventional
filter comprises:
T network means comprising first, second, and third resistive means
forming the left half of the crossbar of the T network, the right
half of the crossbar of the T network, and the leg of the T
network, respectively;
said first and third resistive means comprising said misterminated
impedance means and being connected across said second pair of
electrodes; and
said second and third resistive means comprising the coupling
impedance to said conventional filter.
5. In combination with a crystal filter having a pair of input
terminals and a passband with rounded corners and a terminating
impedance Z.sub.1, a loss equalizer device comprising:
crystal plate means;
at least two pairs of electrodes on said crystal plate with the two
electrodes of each pair being positioned coincidentally on opposite
sides of said crystal plate;
a pair of terminals connected to a first pair of said two pairs of
electrodes; and
two port terminating impedance means having the first port
connected to said crystal filter and presenting an impedance
substantially equal to 2.sub.1 thereto, and the second port
connected to the second pair of electrodes of said loss equalizer
device and presenting an impedance Z.sub.2 thereto, which impedance
Z.sub.2 constitutes a mismatch with said loss equalizer device to
provide a greater loss at the center of said passband than at
points within said passband but near the shoulders thereof.
6. The combination in accordance with claim 5 in which:
said two port terminating impedance means comprises first, second,
and third resistive means forming the left half of the crossbar,
the right half of the crossbar, and the leg of a T network pad,
respectively;
said second pair of electrodes of said loss equalizer device being
connected across first and third resistive means; and
said input terminals of said crystal filter being connected across
said second and third resistive means.
7. The combination in accordance with claim 5 in which:
said pairs of electrodes are positioned a given distance apart and
form a pair of resonators coupled together through that portion of
the crystal plate lying therebetween;
said given distance being sufficient to substantially isolate each
resonator one from the other with respect to electrical coupling
but sufficiently close to permit mechanical coupling between said
resonators.
8. The combination in accordance with claim 7 in which:
said two port terminating impedance means comprises first, second,
and third resistive means forming the left half of the crossbar,
the right half of the crossbar, and the leg of a T network pad,
respectively;
said second pair of electrodes of said loss equalizer device being
connected across said first and third resistive means; and
said input terminals of said crystal filter being connected across
said second and third resistive means.
9. In a system utilizing conventional filter means having input and
output means and a given frequency passband, a loss equalizer
device for equalizing the loss of said conventional filter over
said given passband and comprising:
crystal plate means;
first and second pairs of electrodes coated on said crystal plate
with each pair of electrodes comprising a coated area on
substantially coinciding opposite sides of said crystal plate;
a pair of input terminals connected to said first pair of
electrodes;
a misterminated impedance means having an impedance different from
the midband impedance of said device connected to said second pair
of electrodes to provide greater loss at the center of said
passband than at points within said passband and near the shoulders
thereof;
and means for cascading said loss equalizer device with said
conventional filter means and comprising two port impedance means
constructed to present first and second impedances Z.sub.1 and
Z.sub.to its first and second ports, respectively;
said first port being connected to the output means of said
conventional filter means; and
said second port being connected to the input terminals of said
loss equalizer device and comprising said misterminated impedance
for said loss equalizer device.
Description
This invention relates generally to loss equalization devices for
use with narrowband filters and more particularly to a loss
equalizing device comprising a misterminated monolithic crystal
filter which adds loss to the center of the passband of a
conventional filter with which it is used, and thereby equalizes
the overall loss of the structure to provide a substantially flat
frequency response characteristic over the passband.
One of the primary difficulties in realizing narrowband filters is
the obtaining of a desired flatness of passband. A principal reason
for such lack of flatness lies in the fact that the prior art
filters are designed using insertion loss synthesizing techniques
and utilizing lossless lumped network components. In the actual
construction of such a filter, however, the components employed do
have loss which causes an undesirable displacement of the poles
away from the real frequency axis, with a resulting rounding of the
corners of the passband characteristic. Such roundness of corners
is of sufficient magnitude to prevent the filters from meeting the
specifications of many applications, such as telephone channel
requirements, for example.
There are two approaches widely used today for squaring-up the
corners of the passband. One of these techniques is called
"predistortion." The "predistortion" method of squaring the
passband corners involves the displacing of the pole locations in
the complex frequency plane back toward the real frequency axis
such that the proper-shaped passband is achieved with lossy
components. While this technique does provide an increased flatness
of passband, it also results in a filter with very tight
manufacturing tolerances and one which is quite sensitive to
environmental changes. The aforementioned difficulties involved are
serious enough to raise the manufacturing cost substantially and to
introduce an element of unreliability that in many applications is
not acceptable.
The second technique for equalizing loss is to provide an
additional network in tandem with the filter network which adds
loss to the center portion of the passband in amounts which will
result in a substantially equal loss across the entire passband. It
is a unique application of this latter technique which forms the
basis of the present invention.
It is to be further noted that most prior art loss equalizing
devices are relatively large compared to, and have
frequency-temperature characteristics different from, the filter
they are equalizing.
A primary object of the invention is to provide a simple, reliable
and inexpensive loss equalization means.
A second purpose of the invention is to provide a loss equalization
means comprising a monolithic crystal structure and having the same
temperature frequency characteristic of the filter it is
equalizing.
A third object of the invention is to provide an adjustable loss
equalizing device comprising a misterminated two-pole monolithic
crystal filter.
A fourth object of the invention is to provide a small and compact
loss equalizer device.
A fifth object of the invention is the improvement of loss
equalizers generally.
In accordance with the invention a crystal plate has formed thereon
two pairs of electrodes, which form two resonators coupled together
through the connecting portion of the quartz crystal plate. This
resultant two-pole monolithic crystal filter structure has two
passbands and can be represented by a lattice-type equivalent
circuit. The lower passband is utilized in the present invention
and has a certain image impedance. By terminating the equalization
device in a resistance different from the midband image impedance
the resultant response curve will be caused to have more
attenuation in the passband center than at those frequencies in
that portion of the passband 10 percent to 40 percent out from the
center thereof. B By proper mistermination, the frequency response
curve of the equalization device can be made to approximate a
mirror image of the filter being compensated, i.e., the
conventional prior art filter described above with the undesirable
rounded corners. Thus when the loss equalization device and the
filter are cascaded, the resultant frequency response curve will be
considerably flatter than that of either filter alone.
In accordance with a feature of the invention, the terminating
resistor of the equalizer is made variable so that it can be
adjusted to provide the proper degree of mistermination and the
proper loss characteristic.
The above-mentioned and other objects and features of the invention
will be more fully understood from the following detailed
description thereof when read in conjunction with the drawings in
which:
FIG. 1 is a schematic diagram showing the loss equalization device
of the present invention cascaded with a monolithic crystal filter
whose loss is being equalized;
FIG. 2 is a lattice-type equivalent circuit of the loss
equalization device;
FIG. 3 are curves showing the frequency vs. reactive
characteristics of the series and crossarms of the lattice circuit
of FIG. 2;
FIG. 4 is a curve showing the frequency vs. lattice image impedance
characteristic of the circuit of FIG. 2;
FIG. 4a are curves showing the frequency response curve obtainable
theoretically with lossless components and that obtainable
practically with lossy components;
FIG. 5 is a family of frequencies vs. attenuation curves of the
loss equalization device with various degrees of mistermination;
and
FIG. 6 is a curve of frequency vs. impedance of a combination of
the present invention cascaded with a 10-pole crystal filter.
Referring now to FIG. 1, there is shown a diagram of a structure
including filter 12 comprising 10 resonators formed on a single,
quartz crystal plate 20, a loss equalizing device 10 comprising two
pairs of electrodes 14 and 15 formed on a single quartz crystal
plate 13, and thirdly, a terminating impedance means 11 consisting
of a tunable loss pad having three resistors 16, 17, and 18, with
resistor 16 being adjustable.
The input signal can be supplied at either end of the structure of
FIG. 1, and more specifically to either the terminals 22 or the
terminals 23. The other pair of terminals becomes the output
terminals.
Assume for purposes of discussion that the terminals 22 are the
input terminals. The signal then passes through the loss equalizing
device 10, which is terminated in resistor pad 11. In pad 11 the
impedance presented to the loss equalizing device 10 is determined
primarily by variable resistor 16 and resistor 18. Resistor 16 is
made variable in order to provide a means for tuning the
terminating impedance presented to device 10 to the particular
mismatch required to provide the proper loss equalization
characteristics for any given filter 12. More specifically,
different filters will have slightly different frequency response
characteristics. In order to provide the most optimum loss
equalization for each filter the resistor 16 is made variable.
The impedance presented to filter 12 is determined primarily by
resistors 17 and 18 and ordinarily is equal to the design impedance
of filter 12.
It is to be noted that both the loss equalization device 10 and the
filter 12 are comprised of a quartz crystal plate having a
plurality of pairs of electrodes coated thereon. The energy is
transferred from resonator to resonator through the crystal due to
both the mechanical and the electrical characteristic of the
crystal in a manner well-known in the art.
As discussed generally above, crystal filter 12 is constructed of
components which contain losses and which result in the frequency
response characteristic represented by curve 30 of FIG. 4a, as
compared with the flatter frequency response curve 31 which would
be obtained if the elements of the crystal filter were lossless.
The addition of the loss equalization circuit 10 of FIG. 1 adds
loss to the center portion of curve 30 of FIG. 4a to produce a
substantially flat frequency response curve as shown generally in
FIG. 6.
The operation of loss equalizer device 10 can be better understood
by considering its equivalent lattice-type circuit shown in FIG. 2,
which has the frequency vs. lattice arm impedance characteristics
shown in FIG. 3, and the frequency vs. lattice image impedance
characteristics of FIG. 4. It can be seen from FIGS. 3 and 4 that
the lattice-type network of FIG. 2 has two passbands, the lower
passband occurring between frequencies f.sub.1 and f.sub.2 and the
higher passband occurring between frequencies f.sub.3 and
f.sub.4.
The equations giving the impedance for both passbands of a two-pole
monolithic crystal filter, having the lattice equivalent circuit
shown in FIG. 2, are as follows: f.sub.1 f.sub.4 = f.sub.2 f.sub.3,
by definition (1 )
f.sub.1 f.sub.3 = f.varies.2.sub.ma , by definition (2 )
f.sub.2 f.sub.4 = f.sup.2.sub. mb , by definition (3 ) where
f.sub.ma and f.sub.mb are the midband frequencies of the lower and
upper passbands, respectively.
Then by well known principles:
Where R.sub.oa and R.sub.ob are respectively the image impedances
of the filter at f.sub.ma and f.sub.mb.
Expressions (4 ) and (5 ) for narrow bandwidth filters may be
approximated in the following manner:
The ratio of the two impedances R.sub.oa and R.sub.ob is as
follows:
This results in a ratio of about 700 for a filter having a
bandwidth of 3 kHz. at 10 MHz. When the filter is terminated for
the lower passband, the upper band will not be observable in the
output and need not be considered. It is necessary to consider only
distortion which is induced in the lower passband by the
mistermination.
The amount of such induced distortion as a function of
mistermination is shown in the family of curves of FIG. 5. As the
termination is lowered in value, which increases the mismatch, the
induced distortion is increased. It is apparent from the curves of
FIG. 5 that is is possible to equalize the loss of filters that
have variations in Q merely by trimming the terminating
resistor.
From the foregoing equations and discussions, the following design
procedure can be formulated:
1. Firstly, the amount of equalization required is obtained by an
inspection of the shape of the passband distortion of the filter to
be equalized;
2. From FIG. 3, the data for which has been previously obtained by
testing of a loss equalizer device, the physical parameter required
to obtain the proper equalizer bandwidth is determined;
3. The equalizer is then designed with an impedance suitable for
tandem connection with the filter which is to be compensated;
4. The completed network, including the filter and the loss
equalizer, is then tested to determine if the proper amount of
equalization is accomplished;
5. If the proper equalization has not been obtained, the distortion
of the loss equalizer is tuned by adjustment of a resistor
corresponding to the resistor 16 of FIG. 1, for example, until
optimum flatness of the response curve is obtained.
An example of equalizing loss with the filter of FIG. 1 is shown in
FIG. 6. More specifically, in FIG. 6, there is shown the overall
frequency response curve of the composite filter structure
including a 10 -pole multicrystal filter and a 2 -pole monolithic
crystal filter forming the equalizer. The hatched rectangular lines
40, 41 and 42 of FIG. 6 represent a typical set of specifications
which the frequency response curve must meet, i.e. must fall
within.
* * * * *